It needs work. I am just having trouble picturing it in my mind or how it works or why it was made. Granted some of that stuff, like backstory, isn't needed if you were placing it in a compendium or something, but for building the item and asking opinions it's best if we understand what it's original purpose might have been for.
Did a God give it to a woman (named Pandora?) and say, "Hey... don't drink my beer!"? Naturally, she drank the beer and went on a drunken feminine tirade about all the other women she knew, thus all the evils of alcohol were released into the world, whereas before drinking ale and beer only produced joyous, euphoric happiness in users? I can imagine a woman ruining beer...
Okay, so what we have is basically an animated object that works if you stick a punch card in a slot. It says the barrel has cards and then there's slots for cards. You'll need to list how many cards a 'typical' barrel might be found with and you're going to need a lot more interesting cards.
Let's look at this from a gameplay point of view, not just as a 'this is an item that might be found in a dungeon or castle as a one-shot encounter.'
I would advise making the magic item something like a wondrous figurine in flavor. Give it 1/day activation from a command word and it becomes animated, gaining 4 legs and 2 arms (2 attacks) for 1 minute. As a small construct, I would probably make the natural attack damage 1d6, to be more in line with a short sword or heavier sword from a smaller creature. That's being really generous, because a half-gallon keg is probably a tiny thing. Imagine a gallon of milk, now think about a half-gallon. Sure, the barrel's gonna be more wood and stuff, but it's likely just about as big as a basketball. If you make it tiny, the natural attacks should be 1d4 damage, but then it has to enter squares to attack. That's a big difference in combat.
I would also give it the 'Can't be flanked' quality, since it's a barrel and there's no real discernible front or back.
I would mention that it obeys simple commands from the controller to the best of its abilities; Attack a target, defend a spot, stay, move over there, don't spray fire, etc.
Then mention the 6 card slots and make it more powerful based on if the user has cards in those slots or not. Should be a standard action to change out a card and mention that each card can only be used once per day.
Examples:
- Indigo: If this card is inserted, the barrel's attacks count as ghost touch and the barrel can see invisibility. Multiple: each additional indigo card adds +1d6 damage to the barrel's natural attacks against incorporeal foes. This is extra damage not doubled on a critical hit.
- Orange: If this card is inserted, the barrel gains Resist Fire 5 and its natural attacks inflict 2 additional fire damage. Additionally, if the contents within the barrel are flammable; (oil, alchemist's fire, certain high-proof alcohols) as a free action once per round when animated, the barrel can spray a 15-foot cone of fire dealing 2d4 fire damage (Reflex half). This consumes 1 pint (16 ounces) of stored liquid. Basically you're saying a half gallon barrel, that's 2 quarts in a half gallon, 2 pints in a quart, so 4 uses if the barrel is full. Any contents within the barrel are protected against the adverse effects of heat or fire which might otherwise ruin or harm them (though they aren't kept chilled or refrigerated.) Multiple: each additional orange card adds 5 to the barrel's fire resistance and increases the fire damage from its natural attacks by 2 points. Additionally, increase the Reflex save difficulty by +1 and the fire damage by +1d4 for each extra orange card.
- Green: If this card is inserted, the barrel's movement speed when animated increases by 10 feet and it gains a spider climb effect granting Climb Speed 10 as well as constant feather fall. Multiple: each additional green card increases the barrel's base and climb speeds by 10 feet (counts as enhancement.)
- Red: If this card is inserted, the barrel gains 1 additional weapon arm as a primary weapon.
Multiple: each additional red card increases the barrel's attacking arms by 1.
- Blue card: If this card is inserted, the duration the barrel can remain animated increases by 1 minute per day. This duration need not be continuous, the duration may be split up by the controller, though each activation uses a minimum duration of 1 minute. Multiple: each additional blue card increases the barrel's animation time by 1 minute.
- Yellow: If this card is inserted, the barrel stops moving and emits a 60-foot radius alarm effect (silent, to controller). Any creature with hostile intentions towards the controller entering this radius will automatically trigger the animation of the barrel even if its activation time has been used for the day. This activation time lasts for 1 minute and is increased by any blue cards already slotted. The duration of this effect is 4 hours and it does not function or trigger against creatures already within the radius when the yellow card is inserted. The barrel must remain stationary, it may not be picked up and carried. Attempting to move the barrel cancels its protection and counts as a use of the card. Multiple: Each additional yellow card increases the barrel's protection duration by 2 hours and grants a cumulative +1 to the initiative checks of the controller and any allies within the radius (including the barrel) while its animated by this effect.
- Violet: If this card is inserted, the barrel's hardness increases by 1 and its hit points increase by 5. It gains a +1 resistance bonus on all saving throws. Additionally, once per day if the barrel's hit points drop below 50%, it is affected as though by a mending (whether animated as a construct or not), repairing 1d4 damage. If the barrel would otherwise be destroyed, it is instead left with 1d4 hit points. Multiple: each additional violet card increases the barrel's hardness, hit points, and resistance bonus. Each extra card grants an additional occurrence of damage repair if the barrel drops below 50% hit points or would be destroyed.
Something like that, where at least the barrel is useful even without cards.
